1. Field of the Invention
The present invention relates in general to a method and apparatus to mitigate trapped pressure in a wellhead and in particular to a compressible pressure limiting device for limiting pressure from a void typically located between two crown plugs in a wellhead tree system.
2. Brief Description of Related Art
A horizontal subsea tree has a production outlet extending generally horizontally, in relation to the wellbore, and a bore that is axially aligned with the wellbore. A tubing hanger lands in the horizontal tree and supports a string of tubing extending into the wellbore. The tubing hanger has a vertical passage and a lateral passage extending from the vertical passage and registering with the production outlet of the tree. In some installations an internal tree cap lands in the tree above the tubing hanger, the tree cap normally having a vertical passage that aligns with the vertical passage in the tubing hanger. As a dual safety barrier, a wireline deployed crown plug is installed in the vertical passage of the tubing hanger and another crown plug is installed in the vertical passage of the tree cap. In other installations, the internal tree cap is omitted. In that case, the vertical passage of the tubing hanger is typically plugged with two crown plugs to meet requirements of having dual safety barriers.
Fluid, such as, for example, completion fluid, may be trapped in the vertical passage between the two plugs. The fluid may be relatively cold when it is trapped because the subsea temperature is relatively cold. During production, the well fluid flowing through portions of the wellhead is at a higher temperature and subsequently heats the subsea wellhead. As the fluid trapped between the crown plugs heats up and is restricted from expanding, the trapped fluid pressure can potentially increase above the working pressure of the crown plugs and, thus, damage the integrity of the crown plugs. It is thus desirable to limit the pressure in the void between the crown plugs, without releasing the fluid trapped between the plugs into the environment.